Two-phase (e.g., gas-solid, gas-liquid) flows are ubiquitous across a broad range of manufacturing, agricultural, medical and military applications. In many instances, the flow is intentional and controlled, as in pneumatic conveyors, pesticide applicators or drug delivery devices. In other applications, a gas contaminated with entrained particles/droplets must be refined, or particles/droplets entrained in a gas must be recovered for some further use. As used herein, a “particle” refers to both solid particles and liquid droplets.
Across these applications, there is often a requirement to continuously and reliably separate particles from the entraining gas. A subset of this separation requirement is to classify the removed particles by characteristics, such as size, density, morphology, etc. A variety of structural filters and mechanical separators have been developed for these purposes and, in most applications, they meet or exceed performance requirements.
“Structural filters” refers to passive devices that retain particles in restricted passages or on large surface areas. Screens, membranes and paper or fiber filters are examples of this category. Structural filters have a capacity limit in terms of total allowable mass of captured particles. Once the limit is reached, it must be cleaned or replaced.
“Mechanical separators” refers to devices that induce a differential velocity or vector between the gas and particle. These include devices either with moving parts (such as rotors) or without them (such as vortex or electrostatic or magnetic units). Most mechanical separators continuously discharge particles, so they avoid the mass capacity limitation of structural filters. However, these devices have inherent restrictions on the maximum concentration of particles (mass per unit volume of gas) which they can efficiently process.
Since mechanical separators depend largely on gravity flow, their chamber geometry must address the collected particles' angle of repose and minimum allowable throat dimensions to prevent clogging and bridging within the device. Therefore, angles are typically steep, throats wide and orientation almost always vertical. The configuration of these systems, and the turbulent flows within them, often results in re-entrainment of separated particles, especially under conditions of high loading or contained nano-particles.
In applications where either particles are very fine or the particle loading is quite high, or both, structural filters and conventional mechanical separators may not be economical or may fail altogether. This is becoming increasingly evident in applications as diverse as nanoparticle processing and desert military operations.
Several mechanical separators utilize vortex chambers or electrostatic/magnetic repulsion/attraction to separate two-phase flows into their constituent gas and solid (or liquid) components. These designs typically create a differential velocity and/or vector between the gas and entrained particles. In this manner, particles are concentrated at a collection point and discharged.
However, nano-sized particles are essentially the size of smoke particles, which are easily suspended in flowing gas. Structural filters may be effective in removing nano-particles from air, but there are problems with pressure drop (energy loss), system volume and “harvesting” particles from the filter. Mechanical separators typically create high velocity tangential flows and require particles to travel relatively long distances (several million diameters for micron-sized particles, several billion diameters for nano-particles). Both of these conditions assure that a high fraction of nano-particles will not be separated from the flow.
Several nanoparticle separator systems create, at some point in the process, a two-phase gas-particle flow. Recovering particles from the gas stream may be necessary for consolidation, packaging or other finishing operations. If particles are released to the atmosphere, industrial hygiene requirements often dictate their removal down to a very low concentration in air.
Military operations are completely dependent on equipment that can be quickly degraded by sand and dust. In desert operations, wind-borne sand damages engines, driven components and electrical devices. There is not only a tremendous volume of solids to remove, there is a considerable fraction of solids in the nano-scale range. As a result, structural filters are changed very frequently, with limited success at capturing nano-scale particles. Mechanical filters exhibit two principal shortcomings—poor performance rejecting fine dust particles and excessive wear on fan blades and guide vanes from sand erosion. Personnel safety and hygiene considerations apply here as well; clean air is essential for living, working and medical spaces.
Prior art separator designs have limited ability to process large mass flows having very high solids loading or that contain nano-scale particles. Further, conventional mechanical separators may require large internal volumes or substantial energy to accelerate the gas-solid stream being treated. Relevant information regarding particle separation in a gas stream is disclosed in the articles entitled “Submicrometre Particle Separation Via High-Speed Gas Centrifuges”, Hitchings et al., Proc Instrn Mech Engrs Vol. 211 Part E, pp. 17-29 (1997), and “Process Synthesis For Particle Separations Using Centrifuges”, Agena et al., Computers Chem. Engng Vol. 22 No. 3, pp. 351-356 (1998).
The disadvantages of the prior art are overcome by the present invention, and an improvement and apparatus are hereinafter disclosed for separating particles from a gas stream.